One of the biggest challenges of HTML developers, probably the biggest challenge is browser compatibility. At Ironworks, we’re often asked to produce pixel perfect representations of the original designs, in multiple browsers - especially IE6. In my experience using something like IE tester, or for IE7 and IE8 using the developer toolbar simulated versions aren’t always accurate. Some people go the route of using Adobe’s Browser Lab .
If you need to be sure however, the best bet is to use the real thing. Microsoft makes it easy for you to do just that by offering many flavors of their operating systems and stock browsers. You can see the selections at their IE Application Compatibility VPC Page. Here’s what you can check:
IE6 on XP Service Pack 3
IE7 on XP Service Pack 3
IE8 on XP Service Pack 3
IE7 on Vista
IE8 on Vista
The downside? – you have to re-download the virtual machine every few months. Worth it if you just have to know.